A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning structure, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) or a metrology or inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.
The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.
The term “patterning structure” used herein should be broadly interpreted as referring to a structure that can be used to impart a beam of radiation with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the beam of radiation may not exactly correspond to the desired pattern in the target portion of the substrate. Generally, the pattern imparted to the beam of radiation will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.
A patterning structure may be transmissive or reflective. Examples of patterning structures include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions; in this manner, the reflected beam is patterned.
The support structure supports, i.e. bears the weight of, the patterning structure. It holds the patterning structure in a way depending on the orientation of the patterning structure, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning structure is held in a vacuum environment. The support can be using mechanical clamping, vacuum, or other clamping techniques, for example electrostatic clamping under vacuum conditions. The support structure may be a frame or a table, for example, which may be fixed or movable as required and which may ensure that the patterning structure is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning structure”.
The term “projection system” used herein should be broadly interpreted as encompassing various types of projection system, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate for example for the exposure radiation being used, or for other factors such as the use of an immersion fluid or the use of a vacuum. Any use of the term “lens” herein may be considered as synonymous with the more general term “projection system”.
The illumination system may also encompass various types of optical components, including refractive, reflective, and catadioptric optical components configured to direct, shape, or control the beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”.
The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.
The lithographic apparatus may also be of a type wherein the substrate is immersed in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. Immersion liquids may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the first element of the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.
It may be desirable to project the beam of radiation imparted with a pattern in its cross-section accurately to the substrate (e.g. with respect to one or more features or process layers already formed on the substrate, or with respect to a predetermined characterization of the substrate surface). In such cases, it may be desirable to know the relative position of the substrate with respect to the reticle, in order to position the substrate in the focal plane of the projection system that is located optically between the reticle and the substrate. Therefore, it may be desirable to measure accurately the position of the substrate. This measurement may for instance be done by, in an operation, determining the position of the substrate with respect to the substrate table carrying the substrate. In another operation, the relative position of the substrate table is determined with respect to the reticle. These two measurements together can be used to compute the relative position of the substrate with respect to the reticle. However, it will be understood that several other strategies may be used to determine the relative position of the reticle with respect to the substrate, for instance by directly determining their relative position or by determining the relative position of the substrate with respect to the reticle table. It is implicit in these strategies that the position of the substrate is determined with respect to the position of another object (for example, the substrate table, the reticle table, or the reticle).
Several methods are known to a person skilled in the art to determine the relative position of a substrate with respect to another object. For instance, in case the relative position is determined with respect to the substrate table, the substrate and the substrate table are both provided with alignment markers. The substrate may be provided, for example, with up to 30 alignment markers. First the positions of some or all of the alignment markers on the substrate and the substrate table are determined. This procedure may be done by providing an alignment beam to a first alignment mark. The first alignment marker is positioned in the alignment beam by moving the substrate table, while monitoring the position of the substrate table with interferometric devices. By performing measurements to the diffraction pattern generated by the alignment marker in combination with the alignment beam, the position of the substrate table for which the alignment marker is optimally positioned with respect to the alignment beam can be determined. This operation may be done for some or all of the alignment markers (on the substrate as well as on the substrate table). By comparing the readings of the interferometric devices monitoring the position of the substrate table that correspond to different positions of the alignment markers, the relative position of the substrate with respect to the substrate table can be determined.
However, any other known method may be used to determine the relative position of a substrate with respect to another object. Most of these alignment techniques use alignment markers provided on the substrate. Therefore, use of such techniques may require that alignment markers are provided on the substrate. Such measuring markers may be provided to the substrate by projecting an alignment marker pattern to a layer of resist provided on top of the substrate.
After the exposure, the alignment markers are latently present, and the substrate is transported out of the lithographic exposure apparatus to a place where the latent alignment markers can be made visible, i.e. the alignment marker is made detectable for the alignment arrangement used. This is usually done in a track, as will be known to a person skilled in the art. In such a track, a post exposure bake (PEB) may be carried out in which the substrate is heated to a certain suitable temperature in order to make the latent alignment marker visible, as will be known to a person skilled in the art. After this, the substrate may be transported back into the lithographic apparatus where the alignment markers can be used for determining the relative position of the substrate as described above.
Transporting the substrate out of the lithographic apparatus to the track, where the latent markers are made visible, and transporting the substrate back in to the lithographic apparatus is a time-consuming process and therefore reduces the throughput of the system. This transportation process may also lead to inaccuracies, since the substrate is removed from the substrate table and repositioned on the substrate table after treatment in the track. For example, it will be understood that the substrate may not be in the exact same position with respect to the substrate table after repositioning as it was before removal from the substrate table.